On The Trails: Fall comes; sexton beetles

A burying (or sexton) beetle carries phoretic mites. (Photo by Bob Armstrong)

By Mary F. Willson

Autumn crept up on us, deluding us with a series of wonderfully sunny, summery days in late August and early September — although those days were getting shorter and shorter.

On my pond, three female mallards consort daily — oh, wait…one of them is starting to get dark chest feathers and the top of its head is a bit dark too — clearly a male just starting to get dressed for the mating season to come. Squirrels drop lots of resin-sticky cones, to be collected and stored for later seed-eating. Cottonwoods release a few golden leaves. Fireweed has gone to seed and, in some places, has already let the breezes waft away the fluff-bearing seeds.

Sadly (for me), on Labor Day I saw a couple of bang-bang guys going home from the wetlands with at least six ducks and a goose dangling from hands and backpack.

Along various trails, late-blooming dandelions attract an assortment of small flies. Purple-flowered wandering daisies (Erigeron peregrinus) are common, although I didn’t see much insect visitation. By the way, they can be distinguished from a similar Aster species by looking at the involucral bracts at the back of the purple inflorescences: in asters, those bracts make a cup-like shape, but in daisies they are more spread out. Here and there, we find little stands of an exotic geranium called herb robert, with small, pinkish flowers; pollinated flowers exhibit a tall style on the central ovary that is reminiscent of our wild geranium.

Jewelweed flower bears nectar in the little twist at the back end. (Photo by Bob Armstrong)

On some trails we see yellow-flowered jewelweed (Impatiens noli-tangere). That flower has an unusual construction. The back part of the flower is made of sepals converted to look like petals, and that’s where the nectar is — in the little twist at the end. Petals form the front part of the flower. The scientific name and an alternative common name of touch-me-not reflect the unusual behavior of the seed pod: it suddenly pops open when touched, thus scattering the seeds. Our field guides treat this species as native, but some other sources disagree. Another species of the same genus is truly non-native; it has large purplish flowers of similar construction; a big stand of it grows at the start of the Industrial Boulevard wetland trail. 

Other signs of the changing season are the flocks of birds foraging for insects in the foliage. Recently we watched a flock of mostly golden-crowned kinglets flitting about very actively; another flock was harder to see in the dense foliage but held a distinguishable brown creeper. It’s also time for porcupines to be more active, as their mating season approaches. 

Along the rivers, fish carcasses have been accumulating. This is an annual situation when some of the up-running salmon don’t make it and the successful spawners eventually die. But I wonder if all that wonderful, sunny and dry weather we so enjoyed might have led to unusually low water and caused more fish in those early runs to fail in their spawning run. All those carcasses are fine foraging for blowfly maggots (good bird food!) and other scavengers too. 

A burying (or sexton) beetle explores the orbit of a salmon carcass. (Photo by Bob Armstrong)

An interesting scavenger is known as the burying beetle or the sexton beetle. Although they often bury a dead mammal or bird, they also use dead fish. Male beetles compete with other males for space on a fish, and females compete with other females. When the prize is a whole fish, they don’t bury it, but rather burrow in and under the body. After eggs are laid, both parents may feed the brood, although the chief role of the males may be to defend the resource against other small scavengers.

These beetles are commonly accompanied by tiny mites that ride on their backs, sometimes many dozens of them. The mites are hitchhiking and will lay their eggs in the carcass too. Their ecological role varies with the species: sometimes they benefit the beetles by killing off other scavengers such as nematode worms and fly larvae, but at very high densities they may sometimes also eat beetle eggs.

I was fascinated to watch a mite-loaded sexton beetle crawling around in the eye-orbit of a salmon carcass, looking for just the right place to eat and lay eggs. More extensive essays on sexton beetles and their mites can be found on my searchable website: onthetrailsjuneau@wordpress.com.

• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent.